Synergistic effects during co-pyrolysis and co-gasification of polypropylene and polystyrene

The thermal conversion of waste plastic products can provide a promising solution to the issues of environmental pollution, waste management, and energy needs. However, our present understanding on the thermal reforming of different kinds of plastic wastes mixtures during their co-pyrolysis and co-gasification for syngas production and energy recovery is insufficient. Non-selective urban polypropylene (PP) and polystyrene (PS) waste plastics were chosen for the investigations reported here. Thermal degradation properties of PP and PS as well as their blends in different mass ratios were investigated via thermogravimetric (TG) analysis. Co-pyrolysis and CO2- assisted co-gasification of PP-PS blends were then conducted using a fixed-bed reactor at a temperature of 1173 K. Experimental results from the blends were then compared with the weighted value calculated from their component feedstocks to quantify the degree of synergistic effect. TG results showed that the co-treatment of PP and PS enhanced the respective devolatilization process. The co-pyrolysis of PP and PS enhanced their thermal cracking, leading to synergistic increases in the yields of H2, light hydrocarbons (HC) and total syngas. In co -gasification, the reforming reaction involving CO2 was synergistically enhanced, resulting in the increased yields of H2 and HC. The gasification reactivity of carbon black improved during the co-processing of PP and PS due to the synergistic effect between the two different kinds of plastics. Higher synergistic effects were observed from the plastic mixtures having higher PS content that resulted in increased yield of syngas. Co-gasification of PP-PS blends having 40% PP content (2P3S) exhibited the maximum synergistic effect on H2, CO, and total syngas showing increased yields by 88.8%, 77.7%, and 74.2% respectively. The lowest synergistic effect was observed for HC that showed increased yield of only 25.1%. The optimal CO2 consumption was also observed from the co-gasification of 2P3S. Each gram of 2P3S feedstock consumed approximately 1.80 g of CO2 to result in highest recovered energy of 27.49 kJ/g and the maximum overall energy efficiency of 43.2%. The results revealed that enhanced thermal cracking and CO2 reforming reaction, along with the improved reactivity of carbon black were mainly responsible for increased yields of H2, HC, CO and total syngas during co-gasification. This study contributes to the fundamental understanding of co-processing of non-separated waste plastics for enhanced syngas production and energy recovery, elucidating the synergistic effect between the various kinds of plastics during their co-pyrolysis and co-gasification.